The present invention relates generally to methods and apparatus for making prosthetic devices for humans and specifically prosthetic socket preforms, prosthetic sockets and prosthetic socket attachment components. More particularly, the present invention concerns the fabrication of prosthetic socket preforms of memory plastic material using a blowmolding technique and the heat- and vacuum-forming of such preforms onto a positive cast of a residual limb to form a prosthetic socket. Further, the invention relates to a prosthetic socket attachment component molded within such a prosthetic socket for joining together a limb sleeve, the prosthetic socket and an artificial limb shaft.
Often, a prosthesis is required to assist in restoring an amputee's ability to accomplish many of life's daily tasks. In the case of lower extremity amputations such as the loss of a leg, the prosthesis allows the amputee to stand, walk and run by providing a mechanical extension to the residual limb or stump. Such a prosthesis may comprise an artificial foot connected to an artificial limb shaft with a custom-fitted socket and an elastic sleeve at one end which fits over the residual limb for securing the socket to the residual limb. One of the most important aspects of these prostheses is socket design, the socket being the load-bearing interface between the residual limb and the mechanical support system.
Because each socket must be custom-fitted to the individual patient, socket design and fabrication have been heretofore quite expensive and time-consuming. One technique commonly used to make prosthetic sockets has been to drape a sheet of heated plastic over a positive cast of the residual limb, manually forming the plastic to conform to the contours of the cast. This technique, known as "drape-forming" usually requires two persons and a large degree of manual skill. Further, it often results in significant amounts of wasted plastic and nonuniform forming.
Another technique previously disclosed has been to provide a generally conical, hollow socket preform of memory plastic material which is then placed over a positive cast of the residual limb, the combination being placed in an oven and heated sufficiently to induce shrinkage of the preform onto the cast surface to create a semi-finished socket. This technique, known as "shrink-forming," takes advantage of the characteristics of such memory plastic materials, in particular their tendency to return to their original shape when reheated. This method has been described in an article entitled "Memory plastics for prosthetic and orthotic applications," A. Coombes, C. Greenwood, Prosthetics and Orthotics International, v. 12, pp. 143-155 (1988) and also in "A Computerized System to Manufacture Prostheses for Amputees in Developing Countries," N. Walsh, J. Lancaster, V. Faulkner, W. Rogers, Journal of Prosthetics and Orthotics, v. 1, n. 3, pp. 165-181 (April, 1989). The contents of these two publications is incorporated herein by reference.
Although shrink-forming offers a more efficient, less labor- and skill-intensive method of socket fabrication than drape-forming, the basic method as described above suffers from several problems. First, the preform may develop imperfections or even perforations during heating at the points of initial contact with the positive cast. This is due to the temperature differential between the cast and the preform during heating, with the cast acting as a heat sink at these points of initial contact. Second, the conventional convection-type ovens presently used in shrink-forming result in nonuniform heating of the preform which in turn results in nonuniform shrinkage of the preform over the cast. Finally, the degree of shrinkage of presently available preforms is less than optimal for these purposes, although this deficiency may be somewhat compensated for by applying a vacuum to the preform/cast interface to help conform the preform to the contours of the cast.
It is believed that the root of the last-mentioned problem lies not so much with the shrink-forming technique but rather with the preform itself. A particularly advantageous method of producing the socket preform utilizes blowmolding, offering fewer steps than other techniques such as injection molding. The use of blowmolding to form socket preforms is also disclosed in the above-referenced articles. Blowmolding in such an application involves extruding heated plastic through a mandrel and die to form a parison or tube, enclosing the parison within a mold, and injecting air into the parison until it expands to conform to the contours of the inner cavity of the mold. The use of standard blowmolding techniques, however, does not produce a preform product with optimal shrinkage characteristics. Further, standard blowmolding molds often produce an undesirable raised portion or rib on the inner surface of the preform at its upper end. Because of its location, this rib is difficult to remove and often creates a non-uniform area in the final product. Finally, conventional molds also suffer from parison severing at the upper mold interface during blowmolding of plastics that have a high melt flow index.
One further problem affecting presently available socket technology relates to the mode of attaching the socket and the residual limb sleeve to the artificial limb shaft. In a typical configuration presently in use, the limb sleeve and socket are attached to the socket through a sleeve attachment device molded within the socket. A separate socket attachment plate is then molded or otherwise attached to a holding frame for the socket. This plate has a male or female attachment means that mates with artificial limb shaft. This configuration, requiring multiple parts and multiple assembly steps, adds additional expense to the already costly prosthesis and increases the likelihood of component failure under repeated loads.
Accordingly, it is desirable to provide a method and apparatus for reliably producing a socket preform with improved geometry and shrinkage characteristics. It is further desirable to provide a method of forming sockets from preforms which avoids the problems of nonuniform heating and heat-sinking by the cast. Finally, it is desirable to provide a simpler and more cost-effective means for attaching the prosthetic socket to the residual limb sleeve and artificial limb shaft.